The present invention relates to a method and a device for controlling a vehicle.
U.S. Pat. No. 5,129,496 describes an automatic locking brake. When the brake pedal is actuated and the speed of the vehicle is less than a very small speed value, the braking pressure is locked up in the wheel brakes, i.e., maintained constant, by switching a valve, and is possibly additionally built up by actuating at least one pressure-generating means (pump). The locked-up braking pressure is reduced only when the driver starts off from rest.
German Patent No. 196 21 628 also describes an automatic locking brake (or hillholder) which, when the brake pedal is actuated and the vehicle is detected to be at a standstill, maintains, or possibly increases the braking force input by the driver. This function is deactivated when the driver releases the brake pedal.
In both approaches, the described function is activated as a function of at least the traveling speed, which is generated on the basis of at least one wheel speed signal. Therefore, it is problematic if a vehicle is erroneously inferred to be standing, for example, when the at least one wheel, whose speed signal is the basis for generating the traveling speed signal, locks up. In connection with the described locking brake function, the locked-up braking pressure or the maintained braking force are not reduced anymore. In corresponding friction coefficient conditions between the road surface and vehicle in the short term, or in the case of a large road gradient also in the long term, this results in that, in the extreme case, the vehicle is no longer steerable since the wheels lock up and the driver cannot reduce the braking force at these wheels anymore. This danger exists primarily if the reduction in braking force of the automatic locking brake is performed only after a preselected time elapses subsequent to the release of the brake pedal, or especially, if the deactivation is performed only during starting from rest. The above-described driving situation is particularly critical if it arises in speed ranges, for example speeds lower than 8 km/h, in which an anti-lock controller does not intervene.
An object of the present invention is to improve an automatic locking brake function or a hillholder with respect to the above-described situation. A further object of the present invention is to provide measures that allow the skidding of a vehicle to be estimated from the vehicle behavior.
In an advantageous manner, an automatic locking brake function and/or a hillholder function, which is activated, inter alia, when a vehicle is at a standstill, is improved in that the function is deactivated immediately in response to detected skidding of the vehicle to restore the steerability of the vehicle.
It is advantageous that, when a danger of skidding exists and the brake pedal is released while the function is activated, brake pressure is immediately reduced in at least one wheel while at other vehicle wheels, which are not affected by the reduction in braking force, the braking force is increased so that the automatic locking brake function or the hillholder function is not impaired in the case that an actual skidding does not exist.
In a simple and reliable manner, to detect the skidding, braking force is reduced, for example, at one wheel upon the release of the brake pedal, and it is checked whether the one wheel starts up again, i.e., whether its wheel speed increases. If this is the case, then a skidding situation of the vehicle exists that results in an immediate deactivation of the hillholder or the automatic locking brake function.
In another embodiment, skidding is advantageously detected in that, in the case of an automatic locking brake function or hillholder function, not all the wheels of the vehicle are braked. In this manner, it is possible to check only the starting up of the unbraked wheels to detect skidding.
The estimation of a danger of skidding is performed without any additional sensor technology. A danger of skidding is advantageously assumed when an estimated coefficient of friction falls below a limiting value. In this context, the coefficient of friction is estimated on the basis of quantities representing braking forces in response to a decrease of at least one wheel speed. Extensive sensor technology is not necessary.
The road gradient may be taken into account during the determination of the danger of skidding. Along the lines of the above, the road gradient is ascertained on the basis of signals representing braking forces (e.g., brake pressures) and a vehicle deceleration quantity. In this manner, the accuracy of skidding danger detection is improved.
A skidding danger may be assumed when the estimated coefficient of friction is smaller or only slightly greater than the required coefficient of friction, which may be ascertained as a function of the estimated gradient. Additionally or alternatively, skidding danger may always assumed when a preselected friction coefficient level fails to be met again, independently of the gradient so that skidding danger is always detected even in difficult conditions.
In another embodiment, as an alternative or in addition to the above measures, a skidding danger may always be inferred when a traction control occurs at low speeds including a subsequent vehicle standstill.
In an advantageous manner, the described procedure is not only implemented in connection with a hydraulic braking system but also with an electrohydraulic, an electropneumatic for an electromotive braking system.
It is expedient for the reduction in braking force, for detection of skidding during the existence of a danger of skidding to be performed not as a function of the release of the brake pedal, but to have already been performed at the beginning of the activation of the function during the vehicle standstill. If skidding is detected, then the driver""s desire is given priority, and the braking force input by the driver is taken as a basis for the control of the wheel brake, i.e., the locking brake function or the hillholder are deactivated.
It is advantageous if, to detect skidding, braking force is reduced not only at one but at several wheels, for example, at an entire axle. In this case, it is an advantage that this measure, for example in the case of a hydraulic braking system equipped with a driving dynamics control, takes place silently.